The polyisocyanate mixtures of the diphenylmethane series resulting from the condensation of aniline and formaldehyde in the presence of acid catalysts followed by phosgenation of the resulting polyamine mixtures, contain, in addition to the isomers and higher homologues of diisocyanatodiphenylmethane, varying quantities of impurities, for example, of monoisocyanates, colored impurities and compounds having organically-bound chlorine. Some of these latter compounds contain the chlorine in a more-or-less readily-separable form, called hydrolyzable chlorine, although these compounds are present with compounds having less readily-separable chlorine, also. It is known that these different chlorine compounds influence the reaction of isocyanates with polyols to produce polyurethanes, and that they act in particular, on the rate of the reaction between the isocyanates and polyols. It is particularly desirable in the case of 4,4'-diisocyanatodiphenylmethane, which is used in large quantities for the production of elastic polyurethanes, to limit the effect of the organic chlorine compounds influencing the polyurethane reaction, or, at least, to standardize the effect by controlling the chlorine content of these products. Thus, the content of chlorine in these compounds is an important parameter for the purity of 4,4'-diisocyanatodiphenylmethane.
It is also desirable and necessary to reduce the amount of monoisocyanates, such as phenyl-isocyanate which originates from the residual contents of aniline in the polyamine mixtures used in the phosgenation process to as low a level as possible. This is because phenylisocyanate raises problems of working hygiene during the processing and handling of the isocyanates of the diphenylmethane series, due to its low vapor pressure and high toxicity.
A number of processes are known for removing these impurities in order to attain the required degree of purity of the diisocyanatodiphenylmethanes. It has been proposed, for example, to convert the compounds containing chlorine into almost nonvolatile forms by adding certain substances (for example, see German Pat. No. 1,138,040), then to remove the impurities. Another proposal (German Offenlegungsschrift No. 1,938,384) is based on the use of a complex commercial crystallization process. Also, processes for the distillative purification of diisocyanatodiphenylmethanes have been widely used on a commercial scale.
In the known distillation processes (see, for example, German Auslegeschriften Nos. 2,631,168 and 1,923,214 and U.S. Pat. Nos. 3,892,634 and 3,471,543), the diisocyanatodiphenylmethane isomers are usually separated from their higher homologues in a first distillation step. In a second step, the isomeric diisocyanatodiphenylmethanes are separated, with the more readily-volatile 2,2'- or 2,4'-isomers accumulating as head products and the 4,4'-isomer remaining as the sump product. The 4,4'-diisocyanatodiphenylmethane, which is substantially freed of isomers, is then distilled off again in a final distillation to remove polymerization products which have formed due to the thermal effects of the distillation process.
Another separating step may be added in each case for separating readily- and much less volatile impurites, according to German Auslegeschrift No. 2,631,168. This process allows low chlorine contents to be obtained in the end product when there is a low content of chlorine compounds in the isocyanate mixture which is fed into the distillation. The process is not reliable enough, however, to result in the low chlorine contents required in the end product when there are higher chlorine contents in the original isocyanate mixture.
In order to economize on the number of distillation columns, it has also been proposed (German Offenlegungsschrift No. 2,944,601) to carry out, in the same column, the isomer separation step and the final distillation by side flow removal, with the content of chlorine compounds being reduced by introducing inert gas as an entrainer. This process does not provide a solution to the problem, however, as distillates are obtained which have dimer contents in the vicinity of their saturation limits in diisocyanatodiphenylmethane, i.e., products are produced which are either already cloudy during distillation or which become cloudy shortly thereafter,due to the precipitation of dimers.
A new method for obtaining particularly pure 4,4'-diisocyanatodiphenylmethane is now indicated by the process according to the present invention.